If a pulse generator is connected to a load via a cable, reflections may arise at both ends of the cable causing a pulse to travel forwards and backwards on the cable. It is well known to avoid such reflections by terminating the cable by impedances equal to the characteristic impedance of the cable. This is the so-called matching condition. However, on the load side of the cable matching is not always possible or desirable. If, e.g. the impedance of the load is much bigger than the characteristic impedance of the cable, then there is substantially an open end condition causing a substantially complete reflection of an arriving pulse. That means that the pulse voltage is doubled at this end of the cable. This may be desirable if the load requires a higher voltage than can be delivered by the pulse generator.
Under such an open end condition, and also under any other mismatching condition at the load end of the cable a backwardly travelling pulse is generated which returns to the pulse generator at the other end of the cable. This returning pulse is not disadvantageous per se, however, further reflection at the pulse generator side of the cable must be avoided since it would then interfere with the pulse sequence at the load end of the cable. Therefore, typical output circuits of pulse generators meet the matching condition, i.e. the internal impedance of the pulse generator (or its output circuit) seen from the cable is equal to the characteristic impedance thereof. However, this means that on the input side of the cable one half of the pulse power is consumed by said internal impedance and cannot be used by the load on the other end of the cable.
In some other pulse generators the internal impedance can be switched off. However, this requires a matching condition at the load end of the cable in order to avoid reflections.